diff --git a/Shorewall-init/install.sh b/Shorewall-init/install.sh
index a3bdbd429..5fd9fd533 100755
--- a/Shorewall-init/install.sh
+++ b/Shorewall-init/install.sh
@@ -131,7 +131,7 @@ case "$LIBEXEC" in
 	;;
 esac
 
-if [ -n "INSTALLSYS" ]; then
+if [ -z "$INSTALLSYS" ]; then
     case $(uname) in
 	CYGWIN*)
 	    INSTALLSYS=CYGWIN
diff --git a/Shorewall-lite/install.sh b/Shorewall-lite/install.sh
index 41b5730a0..004a28efd 100755
--- a/Shorewall-lite/install.sh
+++ b/Shorewall-lite/install.sh
@@ -149,7 +149,7 @@ CYGWIN=
 INSTALLD='-D'
 T='-T'
 
-if [ -n "INSTALLSYS" ]; then
+if [ -z "$INSTALLSYS" ]; then
     case $(uname) in
 	CYGWIN*)
 	    INSTALLSYS=CYGWIN
diff --git a/Shorewall/install.sh b/Shorewall/install.sh
index 3a32dc494..4905e8087 100755
--- a/Shorewall/install.sh
+++ b/Shorewall/install.sh
@@ -141,7 +141,7 @@ case "$PERLLIB" in
 	;;
 esac
 
-if [ -n "INSTALLSYS" ]; then
+if [ -z "$INSTALLSYS" ]; then
     case $(uname) in
 	CYGWIN*)
 	    INSTALLSYS=CYGWIN